This application has two goals: 1) to determine how a western style diet (WSD), high in fat and low in Ca and vitamin D3, affects the progression of breast cancer in the PyMT model and 2) to establish whether dietary supplementation with vitamin D3, antibody based neutralization of IL12, or genetic ablation of macrophages, can avert the protumorigenic effects of the WSD. Together, these data will determine how macrophages and macrophage- derived factors contribute to the effects of WSD. The rationale for these experiments is derived from the following facts: First, obesity associated inflammation confers increased risk for several cancers, including breast cancer. Second, it has been established that WSD stimulates hyperproliferation in the terminal end buds of the mouse mammary gland, a cancer-prone region, and that supplementing the WSD with vitamin D3 and Ca markedly suppressed the diet induced hyperproliferation. Third, we found that levels of IL-12, a macrophage-derived cytokine overexpressed in breast cancer, were highly elevated in the serum of mice fed the WSD, and returned to basal levels when WSD was supplemented with calcium and vitamin D3 (Preliminary data). This is of great significance since we recently demonstrated that vitamin D3 interrupts the IL-12 driven cross-talk between macrophages and epithelial cells and our collaborators (Lin and Pollard) established that progression of mammary tumors in the PymT model requires the presence of macrophages. Thus, our hypothesis is that due to the proinflammatory nature of WSD, such a diet will be tumor promoting. We predict that WSD alters the activity of macrophages and therefore has a profound effect on tumor progression in the PymT model of mammary tumorigenesis. We will use three strategies to avert the harmful effects of WSD. 1. We will treat mice with a novel neutralizing anti-IL-12 antibody (provided by Novartis) to determine whether neutralization of IL-12 prevents basal and WSD-induced tumor progression; 2. Based on our findings that vitamin D3, a potent chemopreventive agent, prevents the release of IL-1 from tumor associated macrophages, and that the WSD induced increases in circulating IL-12 are reversed by vitamin D3 (Preliminary data), we will determine whether vitamin D3 abrogates the tumor promoting effects of the WSD. 3. As we hypothesize that the WSD promotes mammary tumorigenesis through stimulation of macrophage-derived factors, we will determine whether the protumorigenic activities of WSD are reduced by genetic ablation of CSF1 in the PymT mice, which results in relative absence of mature macrophages. The data will provide fundamental insight into the mechanisms whereby diet high in fat and low in Ca and vitamin D3 increases the risk of breast cancer and will establish whether nutritional, anti- cytokine, or genetically mediated interruption of a specific cross talk between macrophages and mammary epithelial cells can prevent the harmful effects of WSD on mammary tumorigenesis.